Prior to the present invention, as shown by Holub, U.S. Pat. No. 3,325,450, polysiloxane imides useful as insulation for electrical conductors were prepared by effecting reaction between the diaminopolysiloxane and benzophenonedianhydride in the presence of a suitable organic solvent, such as dimethylformamide, N-methyl-2-pyrrolidone, cresol, etc. The initial reaction was generally carried out from room temperature of 150.degree. C. resulting in the production of an intermediate polyamide acid derivative. Thereafter, the solvent was removed from the resulting amide acid derivative by heating at temperatures of from about 150.degree. C. to 400.degree. C. to effect cyclization and formation of the imide structure.
A similar procedure is shown by Greber, Polykondensationsreaktionen Bifunktioneller Siliciumorganischer Verbindungen, Journal fur praktische Chemie. Band 313, HEFT 3, 1971, S. 461-483, J. A. Barth, Leipzig. Although the procedure of Greber is somewhat different from that shown by Holub, both Holub and Greber utilize a dipolar aprotic solvent, such as dimethylacetamide to form a solution of a silicone-polyamide acid from which films can be cast onto a substrate and further heating is required to effect the cyclization of the polyamide acid to the polyimide state.
Improved results in methods for making silicone-polyimides can be obtained by utilizing aromatic bis(ether anhydride) or the corresponding tetracarboxylic acid in combination with amino alkylene terminated polydiorganosiloxanes as shown, for example, by Takekoshi et al., U.S. Pat. No. 3,833,546 and Heath et al., U.S. Pat. No. 3,847,867, assigned to the same assignee as the present invention.
Although silicone-polyimides have long been recognized for their potential as a source for extrudable wire coating insulation, the flammability requirements of the wire coating industry has generally restricted the use of these materials. In addition to flame retardance, wire coating fabricators also favor extrudable wire coating insulation having at least 150% elongation at break when pulled laterally from a clamped portion of the extrudate along the wire surface. However, efforts to increase the elongation characteristics of silicone-polyimide by increasing the weight percent of silicone has generally been found to increase the flammability of the silicone-polyimide.
The present invention is based on a discovery that silicone-polyimide utilizing aromatic bis(ether anhydride) and amino alkylene terminated polydiorganosiloxane having a critical block length can be extruded onto wire and exhibit an elongation percent of 150 or greater while satisfying UL 94 flammability requirements as defined hereinafter. Wire coating industry requirements can be satisfied providing a critical relationship is maintained between the polydiorganosiloxane block length and the weight percent silicone which is preferably 25% to 45% by weight based on the weight of silicone-polyimide. Polydiorganosiloxane block lengths having an average value of about 20 diorganosiloxy units or less has been found to provide effective results, while a block length of about 5 to about 15 chemically combined diorganosiloxy units is preferred.
As used hereinafter, the expressions "flame resistance", "flammable", "nonflammable" or "flame retardance" with respect to silicone-polyimide means that the silicone-polyimide has satisfied UL 94 V-O requirements for flammability as shown by the Flammability of Plastic Materials Bulletin of Jan. 24, 1980. More particularly, a 5".times.1/2".times.1/8" silicone-polyimide test bar was suspended vertically over a 3/4" Bunsen burner flame as provided in the aforementioned UL 94 test. The test sample exhibited a 94 V-O rating, which includes the following criteria:
A. Not have any specimen which burn with flaming combustion for more than 10 seconds after either application of the test flame. PA1 B. Not have a total flaming combustion time exceeding 50 seconds for the 10 flame applications for each set of five specimens. PA1 C. Not have any specimens which burn with flaming or glowing combustion up to the holding clamp. PA1 D. Not have any specimens which drip flaming particles that ignite the dry absorbent surgical cotton located 12 inches (305 mm) below the test specimen. PA1 E. Not have any specimens with glowing combustion which persists for more than 30 seconds after the second removal of the test flame. PA1 (A) from about 40 to 90% of chemically combined arylimide blocks and PA1 (B) from about 10 to 60% of chemically combined polydiorganosiloxane blocks consisting essentially of from about 3 to about 20 diorganosiloxy units, where the organo radicals attached to silicone are selected from monovalent C.sub.(1-14) hydrocarbon radicals and substituted monovalent C.sub.(1-14) hydrocarbon radicals.
The silicone polyimides of the present invention can be made by effecting reaction between amine terminated polydiorganosiloxane having the formula ##STR1## and organic dianhydrides, as defined hereinafter, where R is a C.sub.(1-14) monovalent hydrocarbon radical or substituted C.sub.(1-14) monovalent hydrocarbon radical, R.sup.1 is a C.sub.(1-14) divalent hydrocarbon radical or substituted C.sub.(1-14) divalent hydrocarbon radical, and n is an integer having an average value of from about 3 to 20 inclusive, and preferably 5 to 15. R.sup.1 is preferably C.sub.(1-14) polymethylene.